1. Field of the Invention
The present invention generally relates to medical cardiac pacers, and more particularly, it pertains to a method for initializing a cardiac pacemakers of the type which responds to the patient's metabolic demand and varies the pacing rate in accordance therewith.
2. Description of the Prior Art
Early cardiac pacemakers provided a fixed-rate stimulation pulse generator that could be reset on demand by sensed atrial and/or ventricular depolarizations. Modern pacemakers include complex stimulation pulse generators, sense amplifiers and leads which can be configured or programmed to operate in single or dual chamber modes of operation, delivering pacing stimuli to the atrium and/or ventricle at fixed rates or rates that vary between an upper rate limit and a lower rate limit.
In recent years, single and dual chamber pacemakers have been developed which measure parameters which are directly or indirectly related to metabolic requirements (e.g., demand for oxygenated blood) and vary the pacing rate in response to such parameters. Such measured parameters include, for example, physical activity of the body, right ventricular blood pressure and the change of right ventricular blood pressure over time, venous blood temperature, venous blood oxygen saturation, respiration rate, minute ventilation, and various pre and post-systolic time intervals measured by impedance or pressure sensing within the right ventricle of the heart. Such sensor-driven pacemakers have been developed for the purpose of restoring rate response to exercise in patients lacking the ability to increase rate adequately by exertion.
In general, a rate responsive pacemaker includes a sensor which produces an output which varies between a maximum sensor output level and a minimum sensor output level ("Sensor Output"), and a pacing rate is provided by the pacemaker ("Pacing Rate") which typically varies as a linear or monotonic function ("f") of the sensor output between a selectable lower pacing rate ("Lower Rate") and upper pacing rate ("Upper Rate"). Function f has a selectable slope (i.e., Pacing Rate change/Sensor Output change) adjustable by means of an external programmer in conjunction with the Lower and Upper Rates. Thus, the Pacing Rate typically provided is equal to the pre-selected Lower Rate plus an increment which is a function of the measured Sensor Output, as follows: EQU Pacing Rate=Lower Rate+f(Sensor Output).
A human's heart rate is normally controlled by a complex set of inputs to the autonomic nervous system. No single type of sensor has been found to be entirely satisfactory for controlling rate response functions.
A significant advantage of the present invention is that each sensor's rate response will be automatically adjusted or optimized, depending upon the current gain setting's ability to achieve a pacing rate which meets the patient's ongoing metabolic needs. A further significant advantage of the present invention is that the weighting of each sensor-determined pacing rate will be automatically adjusted or optimized, depending upon the effectiveness of the sensor gain optimization, such that the pacemaker provides an optimized pacing rate to the patient. A primary benefit which flows directly from the foregoing relates to a significantly reduced need for, and frequency of, re-programming of the pacemaker. Other related benefits include: (1) better accommodation of differences, from patient to patient, in correlations between a particular sensor's output and the corresponding desired pacing rate; (2) better accommodation of differences, as to the same patient over time, in correlation between a particular sensor's output and the corresponding desired pacing rate due to physiological changes of the patient; and (3) better accommodation of differences in correlation between a particular sensor's output and the corresponding desired pacing rate due to device-related behavior, variability in components, sensor drift, etc.
Among the conventional rate responsive pacemakers, those that measure the physical activity of the patient by means of a piezoelectric transducer have become popular among the various rate responsive pacemakers. Such an activity rate responsive pacemaker is described in U.S. Pat. No. 4,485,813 issued to Anderson et al.
Some temperature sensing pacemakers have employed relatively more complex functions to take into account the initial dip in temperature due to the onset of exercise. One such pacemaker is described in U.S. Pat. No. 4,719,920 issued to Alt.
Furthermore, the decay slope of conventional pacemakers do not approximate the heart's normal behavior, in that they are programmed to follow a curve based on a single time constant. This discrepancy between the normal heart deceleration function at the end of physiologic stresses, such as physical activity, and the conventional decay function has not been totally rectified by any pacemaker presently available on the market.
Wherefore, it is desirable to have a new cardiac pacemaker and method of pacing with activity or other rate responsive dependent parameters, for responding to the patient's metabolic demand and for varying the attack and decay pacing rates in harmony with the heart's normal behavior.
At present, many physicians or clinicians set pacing parameters at the time of implant of the cardiac pacemaker, by estimating, somewhat arbitrarily, through successive trials, the approximate settings for these parameters. The numerosity of these parameters and their interactive effect on each other render the optimization process very difficult. As an abridged solution, the physicians resort to selecting extremely conservative parameter settings, even though such selections are not the optimal settings, thus causing a substantial shortening of the pacemaker life, and reducing the patient's safety.
In an effort to minimize patient problems and to prolong or extend the useful life of an implanted pacemaker, it has become common practice in recent years to provide programmable parameters in order to permit the physician to select and adjust the desired parameters to match or optimize the pacing system to the heart's physiologic requirements. The physician may adjust the output energy settings to maximize the pacemaker battery longevity while ensuring an adequate patient safety margin. Additionally, the physician may adjust the sensing threshold to ensure adequate sensing of the heart's intrinsic depolarization of cardiac tissue, while preventing oversensing of unwanted events such as myopotential interference due to upper body movements or electromagnetic interference (EMI).
Recently, rate responsive pacing systems with many programmable variables have been developed and marketed. These systems are based upon sensing a sensor derived variable that is an indicator of the patient's true metabolic and physiologic needs. Similarly, programmable parameters are required to enable to optimize this rate response function.
Wherefore, it is desirable to design a pacemaker and a method of programming the same for automatically initializing the optimal parameter settings either at the time of implant, or thereafter during subsequent follow-ups, with minimal guess or estimation on the part of the physician the pacemaker should more accurately determine the programmable parameter values and reduce the time required to implant or conduct follow-up sessions.
One attempt at optimizing the pacing parameters is described in U.S. Pat. No. 4,867,162 issued to Schaldach. The Schaldach patent generally discloses a cardiac pacer having digital logic circuitry for choosing the characteristics of the pulses to be generated in responses to signals from several physiologic sensors detecting different exercise-related body functions.
The Schaldach pacer uses an external variable for determining the physiological exertion. This external variable is not detectable in the normal operation, but is ascertainable indirectly during the pacing operation from other physiological measured variables. Look-up tables are then used to associate the external variables to the physiological variables.
While this teaching constitutes an improvement over the conventional methods, it has not proven to be completely satisfactory in addressing and resolving the optimization problems associated with the optimization process.
The initialization process disclosed in the Schladach patent is lengthy and somewhat complicated. It generally requires high rate pacing and a substantial memory size for processing the information. Furthermore, the accuracy of the initialization might be compromised due to the extrapolation of the derived data.
Another attempt for automatically adjusting the settings is described in: "Clinical Experience with a New Activity Sensing Rate Modulated Pacemaker Using Autoprogrammability" by V. Mahaux et al., in PACE volume 12, August 1989 issue, pages 1362 through 1368. This article describes the autoprogrammability feature used in the Siemens, Elema AB's Sensolog 703 pacemaker.
The Sensolog 703 pacemaker is a single chamber activity sensing, rate modulated, multiprogrammable pulse generator whose main programmable variables include pacing mode, sensor states, minimum and maximum rates, recovery time and responsiveness. The responsiveness of the pulse generator is determined by two calibration points corresponding to two levels of exercise called "low work" (LW) and "high work" (HW). During the adjustment procedure, the physician defines the desired pacing rates for LW and HW, and asks the patient to perform the corresponding physical activites every thirty seconds. The last sensor output registered at each level of activity is compared to the desired pacing rate by an algorithm in the programmer and optimal sets of slope and threshold values are suggested. The Sensolog 703 pacemaker needs to be manually reprogrammed at various phases after implant, and various tables relating settings to corresponding slope-threshold combinations as well as tables relating rate response to sensor values are also required for programming the parameters.
It is therefore obvious that the Sensolog 703 pacemaker has not demonstrated the ease of use required for an optimal operation of the pacemaker. In fact, the physician's personal interaction is still necessitated at various phases of the automation process. Furthermore, the multi-phase automatization somewhat defeats the object behind the simplification of the operation of the pulse generator, and does not alleviate many of the problems associated with conventional programming methods.
Additionally, the proposed automization method has not attained, nor does it inspire the level of confidence expected from an automized procedure.
Similarly, other pacemakers, such as Medtronic Inc.'s Activitrax II Model 8412-14, Medtronic, Inc.'s Legend Model 8416-18, Cook Pacemaker Corporation's Sensor Model Kelvin 500, Telectronics' Meta MV Model 1202, Cordis Pacing Systems ' Prism CL Model 450A, and Intermedics, Inc.'s Nova MR pacemakers have incorporated the programmability feature of various variables. However, these pacemakers generally require manual programming for entering the values of the desired parameters, in that the operating physicians estimate, through successive trials, the approximate settings for these parameters.
Medtronic Inc.'s Legend and Activitrax II are single chamber, multi-programmable, rate responsive pacemakers, whose rate responds to physical activity. These pacemakers may be programmed to the following parameters: mode, sensitivity, refractory period, pulse amplitude, pulse width, lower and upper rates and rate response gain and activity threshold.
Cook Pacemaker Corporation's Sensor Model Kelvin 500 is a unipolar, multimodal, rate responsive, processor-based pacemaker capable of monitoring the temperature of the blood in the right heart, and making the decision to increase the rate as a result of the patient's physiologic stress. This pacemaker allows for the manual programming of the following parameters: Mode, sensitivity, refractory period, pulse width, lower and upper rates, and interim rate.
Telectronics' Meta MV Model 1202 is an implantable multi-programmable bipolar cardiac pulse generator with telemetry. It can be programmed to operate in one of four pacing modes: demand inhibited (VVI or AAI); asynchronous (VOO or AOO); demand inhibited with an automatic rate response based on sensed changes in respiratory minute ventilation; or adaptive non-rate responsive mode. The following operating parameters are also programmable: Standby rate; sensitivity; pulse amplitude; pulse width; refractory period; minimum heart rate; and maximum heart rate.
Cordis Pacing System's Prism CL Model 450A is a rate responsive single-chamber, multi-programmable, implantable pulse generator with telemetry, for pacing and sensing in the ventricle. The following parameters are programmable: pacing modes (VVI, VVT, VOO); rate response (ON, OFF); electrode polarity; minimum and maximum rates; output current; pulse width; sensitivity; refractory period; and automatic calibration.
The pacer functions described in the Cordis pacemaker manual, are as follows: The target Rate Control Parameter (RCP) is the reference RCP that the pacer uses to control the pacing rate. The pacer determines what the appropriate rate should be by comparing the measured RCP to the target RCP. If the measured RCP is different from the target RCP, rate is increased or decreased until the measured RCP equals the target RCP. The target RCP is a dynamic variable which is first determined by an initialization process, which is automatically activated when rate response is programmed ON. The pacer then continuously makes automatic adjustments to the target RCP to adjust rate response.
The initial RCP is determined while the patient is at rest. During initialization, the RCP is measured for approximately 20 paced cycles to establish the target RCP. If intrinsic activity is sensed during the initialization process, initialization is temporarily suspended and the rate is increased by 2.5 ppm every cycle until pacing resumes. Once initialization is completed and the initial target RCP has been established, rate response is automatically initiated and the automatic calibration function is enabled. The pacer indicates the end of the initialization proces by issuing an ECG signature in the succeeding cycle.
The automatic calibration feature is described in the pacemaker manual as follows: When rate response is ON, the pacer continuously calibrates the target RCP while making adjustments for drifts in RCP that can occur because of lead maturation, drug therapy, and physiologic factors other than those related to emotional stress and exercise. The frequency of adjustment depends, in part, on the speed at which calibration occurs (Slow, Medium, or Fast).
Intermedics, Inc.'s Nova MR is an implantable, unipolar pulse generator designed to provide metabolic response pacing to either the atrium or ventricle. It senses variations in blood temperature and uses this information to vary the pacing rate in response to the patient's metabolic demand. The following functions are programmable to determine the pulse generator's response to variations in blood temperature: Rate response; onset detection sensitivity'; and post-exercise rate decay.
It is therefore abundantly clear that while some of these pacemakers have accomplished satisfactory results, they have not taught a method for simultaneously and automatically initializing optimal parameter settings for sensitivity threshold; pulse amplitude and width; activity threshold; and pressure (dp/dt) rate response gain, either at the time of implant, or thereafter, during subsequent follow-ups, with minimal guess or estimation on the part of the physician.